Polyolefin resins have recently attracted attention as ecofriendly materials in view of the problems of waste plastics disposal and environmental hormones. Specifically, polyethylene resins and polypropylene resins are under study as alternative materials to polyvinyl chloride resins.
However, since polyolefin resins are nonpolar materials, it is very difficult for them to perform such functions as printability, adhesiveness and flame retardancy. In particular, polyolefin resins belong to one of those classes of resins which are highest in flammability, it is a problem difficult to be solved to cause them to perform flame retardancy. Currently, in many instances, this problem is coped with by incorporating some or other halogen-containing flame retardant into polyolefin resins.
The halogen-containing flame retardant is highly effective in rendering materials flame-retardant and decreases the moldability and the mechanical strength of moldings only to a relatively small extent. However, when it is used, a large amount of a halogen-based gas may possibly be generated in the step of molding or upon combustion and it is a matter of concern that the gas generated may corrode apparatus or adversely affect human bodies. Accordingly, a treatment method without using any halogen-containing compound, namely a halogen-free treatment method for rendering materials flame-retardant is strongly desired from the safety viewpoint.
As one of the halogen-free technologies for rendering polyolefin resins flame-retardant, a technology which comprises adding a metal compound which will not generate any toxic gas upon combustion, such as aluminum hydroxide, magnesium hydroxide or basic magnesium carbonate, to polyolefin resins is disclosed in Japanese Kokai Publication Sho-57-165437 and Japanese Kokai Publication Sho-61-36343, for instance.
However, for providing polyolefin resins, which are readily combustible, with a satisfactory level of flame retardancy, it is necessary to add the above metal compound in large amounts. As a result, there arises a problem: the moldings obtained markedly decreases in mechanical strength and can hardly be put to practical use.
Among the metal compounds mentioned above, metal hydroxides such as aluminum hydroxide and magnesium hydroxide, when added to polyolefin resins, cannot form coat layers upon combustion but allow exposure of fragile ashes and dropping of residues. As a result, their function as thermal insulation layers are lost at early stages, and the spreading of fire due to deformation of materials cannot be prevented.
Another method of providing polyolefin resins with flame retardancy has also been proposed which comprises adding a phosphorus-based flame retardant thereto to thereby utilize the oxygen barrier effect produced by surface coat formation upon combustion. However, for providing polyolefin resins, which are readily combustible, with a satisfactory level of flame retardancy, it is necessary to add a phosphorus-based flame retardant in large amounts. As a result, there arises a problem: the moldings obtained markedly decreases in mechanical strength and can hardly be put to practical use.
When a phosphorus-based flame retardant is added to polyolefin resins, it may indeed locally form a coat but cannot form any strong coat layer as continuous layer. The coats formed locally are very weak in mechanical strength and, upon combustion, allow exposure of fragile ashes and dropping of residues. As a result, their function as thermal insulation layers are lost at early stages, and the spreading of fire due to deformation of materials cannot be prevented.
Further, Japanese Kokai Publication Hei-06-2470 discloses a resin composition which comprises a polyolefin resin and, as additives, red phosphorus or a phosphorus compound and a swellable graphite species. This resin composition has sufficient flame retardancy when evaluated from the oxygen index viewpoint and can form coat films only locally but cannot form any firm and continuous coat layer. The coats formed locally are very weak in mechanical strength and, upon combustion, allow exposure of fragile ashes and dropping of residues. As a result, their function as thermal insulation layers are lost at early stages, and the spreading of fire due to deformation of materials cannot be prevented.
Therefore, when used as wall reinforcements in the form of molded flame-retardant sheet materials, for instance, polyolefin resins cannot satisfy the fire resistance or fire protection test requirement that when one side is heated to 1,000° C. the reverse side temperature shall not be not higher than 260° C. Thus, not only the fire resistance is not satisfactory but also there arises a problem: fragile ashes alone remain and residues drop in the fire resistance or protection test, so that their function as thermal insulation layers are lost at early stages.